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I am curious as to why the engines (specifically engines 1 & 4) are positioned far closer to the fuselage on the An-124 compared to a typical 4-engined jet.

I am curious as to whether this design was totally due to wing aerodynamics, or if it may also be related to the shoulder wing design & wing droop when not flying.

enter image description here

Overlay that matches the wingspan of a 747-100 (top) to an An-124, showing the more inward placement of the jet engines.

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    $\begingroup$ @ymb1 no the edits made it WAY more clear! Thanks! $\endgroup$ – chickenricekid Nov 18 '16 at 23:16
  • $\begingroup$ It might actually be a closer comparison to the 747-100 through -400. That image is of the -8, which has a different wing. $\endgroup$ – egid Nov 19 '16 at 6:36
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In general, designers would like the engines to be closer to the fuselage, more like the An-124. With the low wing on the 747, the engines have to be placed far enough out that they can still have enough clearance from the ground. This trades with the height of the landing gear and dihedral of the wing at the fuselage. With a high wing on the An-124 they have plenty of room and can place them closer.

Although the engines do provide some relief to the regular bending loads from lift on the wings, there are lots of reasons to keep the engines closer to the fuselage. Better handling with engines failed is one. But it also affects the amount of wiring and tubing you need to get to the engines, and there are other loads like hard landings that are easier to design for with the engines closer to the fuselage.

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  • $\begingroup$ In that case, why are there not more airplanes designed with engines closer to the fuselage? $\endgroup$ – user6035379 Nov 19 '16 at 8:33
  • $\begingroup$ Rest assured that the existing engine placement was arrived at after lots of studies. This answer gives some reasons. Generally, placing them further outboard enhances the bending relief effect, and ground clearance can be achieved with raising the engine like on the 777 or newer 737s. $\endgroup$ – Peter Kämpf Nov 19 '16 at 9:06
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    $\begingroup$ @user6035379: If you stop reading answers after the first three words, you will never learn why. Too bad! $\endgroup$ – Peter Kämpf Nov 19 '16 at 9:09
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Just a guess: The requirements asked for take-off with two dead engines on one wing. Not fully loaded, but empty except for some fuel to transfer to a safe airbase. The An-124 was developed as a military transport and can operate from improvised airfields. Such a requirement would make sense.

The same requirement existed for the An-70 turboprop transport, but here the inboard engines had to be moved far enough out to allow paratroopers to jump off the plane from the forward doors without ending up in the props. So it is very likely that the same requirement was asked of the An-124.

Why engines are where they are was discussed here.

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    $\begingroup$ @ymb1: Call it as you like: More rudder authority, less yawing moment due to engines, smaller vertical to fulfil all requirements. $\endgroup$ – Peter Kämpf Nov 18 '16 at 22:44
  • $\begingroup$ There's also the structural issues, balancing the bending moments of the lift with the engines when in the air and making sure the wing can support the weight of the engines on the ground. Not caring about noise would also mean engines could be closer. $\endgroup$ – Notts90 Nov 18 '16 at 22:54
  • $\begingroup$ Also the high wing configuration of the AN-124 would better facilitate the engine closer to the fuselage; a similar configuration exists with he Lockheed C-141 and C-5 aircraft as well. So too, does Boeing's C-17 and the Illyushin IL-76. $\endgroup$ – Carlo Felicione Nov 18 '16 at 22:57
  • $\begingroup$ @Notts90: All structural issues are identical for airliners, too. Noise is taken care of by the low wing layout of airliners. No, this does not explain it. $\endgroup$ – Peter Kämpf Nov 18 '16 at 23:12
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    $\begingroup$ @user6035379 It is a solid well educated guess from a professional in the field. $\endgroup$ – ymb1 Nov 19 '16 at 15:17

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